The present invention relates to the recording, storage and reading of magnetic data, particularly rotatable recording media, such as thin film magnetic disks having smooth surfaces for data zone. The invention has particular applicability to high density recording media exhibiting low noise and having improved flying stability, glide performance and head-media interface reliability for providing zero glide hits.
The requirements for high areal density impose increasingly greater requirements on magnetic recording media in terms of coercivity, remanent squareness, low medium noise and narrow track recording performance. Data are written onto and read from a rapidly rotating recording disk by means of a magnetic head transducer assembly that flies closely over the surface of the disk.
It is considered desirable during reading and recording operations to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. This objective becomes particularly significant as the areal recording density increases. The areal density (Mbits/in2) is the recording density per unit area and is equal to the track density (TPI) in terms of tracks per inch times the linear density (BPI) in terms of bits per inch.
The increasing demands for higher areal recording density impose increasingly greater demands on flying the head lower because the output voltage of a disk drive (or the readback signal of a reader head in disk drive) is proportional to 1/exp(HMS), where HMS is the space between the head and the media. Therefore, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head, thereby permitting the head and the disk to be positioned in closer proximity with an attendant increase in predictability and consistent behavior of the air bearing supporting the head.
In recent years, considerable effort has been expended to achieve high areal recording density. Among the recognized significant factors affecting recording density are magnetic remanance (Mr), coercivity, coercivity squareness (S*), signal/noise ratio, and flying height, which is the distance at which a read/write head floats above the spinning disk. Prior approaches to achieve increased areal recording density for longitudinal recording involve the use of dual magnetic layers separated by a non-magnetic layer as in Teng et al., U.S. Pat. No. 5,462,796, and the use of a gradient magnetic layer interposed between two magnetic layers as in Lal et al., U.S. Pat. No. 5,432,012.
However, the goal of achieving a rigid disk recording medium having an areal recording density of about 100 Gb/in2 has remained elusive. In particular, the requirement to further reduce the flying height of the head imposed by increasingly higher recording density and capacity renders the disk drive particularly vulnerable to head crash due to accidental glide hits of the head and media. To avoid glide hits, an accurately controlled movement of the head and a smooth surface of data zone are desired.
It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements, particularly a high-density magnetic rigid disk medium for longitudinal and perpendicular recording. The magnetic anisotropy of longitudinal and perpendicular recording media makes the easily magnetized direction of the media located in the film plane and perpendicular to the film plane, respectively. The remanent magnetic moment of the magnetic media after magnetic recording or writing of longitudinal and perpendicular media is located in the film plane and perpendicular to the film plane, respectively.
In theory, perpendicular media is capable of considerably higher linear data density. Generally, this possibility stems from the fact that information is stored in perpendicular media in discrete domains having opposite magnetization to the magnetization found in the surrounding areas. Such domains can potentially reside in crystals in the media. Typically the information is read from the media through use of a magnetic head that converts local discontinuities present in the discrete domains of perpendicular magnetization into electrical fields which can then be processed as information.
However, between the discrete domains of magnetization, magnetization parallel to the surface of the media, or subdomains or opposite magnetization, are usually present. This is particularly true in situations where remanent magnetization of a layer is significantly smaller than the saturation magnetization of the media. In such situations, the transitions between the domains can cause undesirable electronic signals stemming from, essentially, magnetic noise.
Several terms that are important in describing magnetic recording media are explained below. Coercivity essentially refers to how firmly the media holds a particular orientation of magnetization. For example, how much energy is required to cause a crystal in the media to change orientation. On a magnetization hysteresis (M-H) curve, the required applied magnetic field to reduce the magnetization of the material to zero is called coercivity Hc. Permeability (xcexc) is equal to B/H, where B is the flux density and H is the applied magnetic field.
The easy axis of magnetization of a crystal is the direction of spontaneous domain magnetization in the demagnetized state. The direction of the easy axis of magnetization can be detected on M-H curves. Along the easy axis of magnetization, the M-H curve is forms a square. Along the hard axis direction, the M-H curve is skewed.
Anisotropy refers to the energy stored in a crystal by virtue of the work done in rotating the magnetization of a domain of the crystal away from the easy axis of magnetization. Output basically refers to the strength of the flux created by the media to read the media. Media noise comes from the recording medium. When a magnetic pulse and a transition is written during recording, there is a noise when the signal is being readback.
Some materials change dimension when exposed to a magnetic field. This effect is called magnetostriction. Most NiFe compositions exhibit magnetostriction, except the composition of Ni81Fe19.
A multilayer superlattice has a structure with many interfaces of magnetic/non-magnetic layers. A bilayer superlattice [A/B]n has n bilayers stacked together to form a superlattice, e.g., [Co/Pt]n, [Co/Pd]n, [CoX/Pt]n, [CoX/Pd]n, where X=Cr, B, etc. The thickness of layers A and B can vary from about 3 xc3x85 to about 10 xc3x85 and from about 5 xc3x85 to about 20 xc3x85, respectively.
A soft magnetic layer (also referred as xe2x80x9ckeeper layerxe2x80x9d) is a layer on the substrate of a magnetic recording medium that gives better writing efficiency by pulling the magnetic flux down from the writing pole of a head of the magnetic recording medium. Soft magnetic layers are made of soft magnetic materials. Soft magnetic material is one of the two kinds of commonly available magnetic materials. One kind has a high coercivity and is called hard magnetic material, e.g., CoCr, CoCrTa and CoCrPt. Because it has high coercivity, it is xe2x80x9chardxe2x80x9d to change the magnetization direction unless a strong reverse magnetic field is applied. Another kind is has a very low coercivity in the range of 0.1 Oe to 500 Oe and is called a soft magnetic material, e.g., NiFe, CoZrNb, FeAlNx. Because it has a low coercivity, it is easy (xe2x80x9csoftxe2x80x9d) to change the magnetization direction with a very small reverse magnetic field. xe2x80x9cHardxe2x80x9d and xe2x80x9csoftxe2x80x9d magnetic materials in the context of this invention are not related to mechanical softness or hardness of the material.
In order to undertake perpendicular recording, it is necessary to utilize a magnetic recording media having perpendicular anisotropy. Perpendicular anisotropy is essentially due to a crystal structure of the magnetic material that creates a magnetic moment perpendicular to the surface of the media. One typical perpendicular magnetic material is the alloy cobalt-chromium (CoCr).
A substrate material conventionally employed in producing magnetic recording rigid disks comprises an aluminum-magnesium (Alxe2x80x94Mg) alloy. Such Alxe2x80x94Mg alloys are typically electrolessly plated with a layer of NiP at a thickness of about 15 microns to increase the hardness of the substrates, thereby providing a suitable surface for polishing to provide the requisite surface roughness or texture.
Other substrate materials have been employed, such as glass, e.g., an amorphous glass, glass-ceramic material which comprise a mixture of amorphous and crystalline materials, and ceramic materials. Glass-ceramic materials do not normally exhibit a crystalline surface. Glasses and glass-ceramics generally exhibit high resistance to shocks.
A conventional longitudinal recording disk medium is depicted in FIG. 1 and typically comprises a non-magnetic substrate 10 having sequentially deposited on each side thereof an underlayer 11, 11xe2x80x2, such as chromium (Cr) or Cr-alloy, a magnetic layer 12, 12xe2x80x2, typically comprising a cobalt (Co)-base alloy, and a protective overcoat 13, 13xe2x80x2, typically containing carbon. Conventional practices also comprise bonding a lubricant topcoat (not shown) to the protective overcoat. Underlayer 11, 11xe2x80x2, magnetic layer 12, 12xe2x80x2, and protective overcoat 13, 13xe2x80x2, are typically deposited by sputtering techniques. The Co-base alloy magnetic layer deposited by conventional techniques normally comprises polycrystallites epitaxially grown on the polycrystal Cr or Cr-alloy underlayer. A conventional perpendicular recording disk medium is similar to the longitudinal recording medium depicted in FIG. 1, but does not comprise Cr-containing underlayers.
Conventional methods for manufacturing longitudinal magnetic recording medium with a glass or glass-ceramic substrate comprise applying a seed layer between the substrate and underlayer. A conventional seed layer seeds the nucleation of a particular crystallographic texture of the underlayer.
Conventional Cr-alloy underlayers comprise vanadium (V), titanium (Ti), tungsten (W) or molybdenum (Mo). Other conventional magnetic layers are CoCrTa, CoCrPtB, CoCrPt, CoCrPtTaNb and CoNiCr.
The seed layer, underlayer, and magnetic layer are conventionally sequentially sputter deposited on the substrate in an inert gas atmosphere, such as an atmosphere of pure argon. A conventional carbon overcoat is typically deposited in argon with nitrogen, hydrogen or ethylene. Conventional lubricant topcoats are typically about 20 xc3x85 thick.
Tang et al., U.S. Pat. No. 5,750,270, discloses multi-layer magnetic recording media incorporating a soft magnetic layer for better writing efficiency. Tang et al. discloses that NiFe with very low magnetostriction, low coercivity, and high permeability is one of the preferred materials as the soft magnetic layer for perpendicular recording. The thickness of the soft magnetic layer may require to be as thick as 5000 xc3x85. However, this inventor found that with the conventional sputtering process using Argon sputter gas, the surface roughness increases as the thickness of NiFe increases. In particular, when the thickness of the Argon sputtered NiFe layer is about 5000 xc3x85, the surface roughness of the NiFe layer is high, which causes the top surface of media to also have a high roughness. This in turn could cause head crash due to accidental glide hits of the head and media.
Therefore, there exists a need for technology enabling the use of a structure that could increase the medium coercivity by increasing the interfacial anisotropy and provide a smooth topography of the soft magnetic layer of a recording medium.
During the course of the present invention, it was found that a multilayer magnetic recording medium comprising a multilayer superlattice could have a smooth topography of the soft magnetic layer of a recording medium by using a special process for thick NiFe deposition. This process can reduce surface roughness of thin films remarkably, as well improve the soft magnetic properties of NiFe keeper layer. Therefore, the glide height performance could be improved.
This inventor unexpectedly discovered that the presence of interstitial nitrogen in a soft magnetic layer greatly reduces the surface roughness of the soft magnetic layer as compared to another soft magnetic layer without interstitial nitrogen. xe2x80x9cInterstitial nitrogenxe2x80x9d refers to nitrogen within the interstices of the soft magnetic layer. Interstitial nitrogen is different from nitrogen in the material forming the soft magnetic layer. For example, FeAlNx is a soft magnetic material but nitrogen of FeAlNx is not interstitial nitrogen. Interstitial nitrogen could be nitrogen by itself or nitrogen of a nitrogen-containing material that is not a soft magnetic material.
In particular, with the sputtering gas ratio Ar/Nitrogen at 90:10 (i.e. 10% of nitrogen concentration), the coercivity on the NiFe could be improved from a few hundred oersted (which sputtered with pure argon gas) to only a few oersted in the direction of easy axis of magnetization. The process with Ar/nitrogen mixture gas could also reduce the surface roughness of NiFe film by half.
The present invention is a magnetic recording medium comprising a substrate, a multilayer superlattice comprising a magnetic layer and a non-magnetic layer and a soft magnetic layer comprising interstitial nitrogen interposed between the substrate and the multilayer superlattice.
An embodiment of the present invention is a method of manufacturing a magnetic recording medium, the method comprising sputter depositing a soft magnetic layer comprising interstitial nitrogen on a substrate; and sputter depositing a multi layer superlattice comprising a magnetic layer and a non-magnetic layer on the soft magnetic layer.
Another embodiment of this invention is a magnetic recording medium comprising a substrate; a multilayer superlattice comprising a magnetic layer and a non-magnetic layer and a soft magnetic layer interposed between the substrate and the multilayer superlattice, wherein the soft magnetic layer comprises a means for reducing the surface roughness of the soft magnetic layer. Embodiments of the means for reducing the surface roughness of the soft magnetic layer include, but are not limited to, interstitial nitrogen or any other material in the soft magnetic layer, wherein the other material is capable of causing the soft magnetic layer to have a smooth surface. For example, the other material could be a lubricant or an additive in the soft magnetic layer.
Additional advantages and other features of the present invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present invention. The drawings and description are to be regarded as illustrative in nature, and not as restrictive.